This invention is in the field of measurement techniques, and relates to an optical system and method for the accurate measurement of parameters of regular patterned structures. The invention is particularly useful in controlling a lithography process.
Lithography is widely used in various. industrial applications, including the manufacture of integrated circuits, flat panel displays, micro-electro-mechanical systems, micro-optical systems etc. Generally speaking, the lithography process is used for producing a patterned structure. During the manufacture of integrated circuits, a semiconductor wafer undergoes a sequence of lithography-etching steps to produce a plurality of spaced-apart stacks, each formed by a plurality of different layers having different optical properties. Each lithography procedure applied to the wafer results in the pattern on the uppermost layer formed by a plurality of spaced-apart photoresist regions.
To assure the performance of the manufactured products, the applications of the kind specified above require accurate control of the dimensions of the sub-micron features of the obtained pattern. When dealing with wafers, the most frequently used dimensions are the layer thickness and the so-called xe2x80x9ccritical dimensionxe2x80x9d (CD). CD is the smallest transverse dimension of the developed photoresist, usually the width of the finest lines and spaces between these lines. Since the topography of the measured features is rarely an ideal square, additional information found in the height profile, such as slopes, curves etc., may also be valuable in order to improve the control of the fabrication process.
Generally, an ordinary optical microscope can be used for measuring features"" dimensions. A microscope is practically capable of measuring line width with a resolution of no less than 0.1 xcexcm. The current high-performance semiconductor devices, however, have features"" dimensions of 0.18 xcexcm, and require CD measurement with the resolution of a few nanometers.
Several Optical CD (OCD) measurement techniques recently developed rely on imaging a certain test pattern which is placed in a special test area of the wafer. These techniques utilize various methods aimed at amplifying tiny differences in the line-width to obtain macroscopic effects that could be resolved by visible light, although the original differences are more than two orders of magnitude below the wavelength used. However, some of these techniques do not rely on fundamental physical effects, and thus could be more effective in some cases and less effective in others.
Another kind of technique utilizes scatterometric measurements, i.e., measurements of the characteristics of light scattered by the sample. To this end, a test pattern in the form of a grating is usually placed in the scribe line between the dies. The measurement includes the illumination of the grating with a beam of incident light and determining the diffraction efficiency of the grating under various conditions. The diffraction efficiency is a complicated function of the grating line profile and of the measurement conditions, such as the wavelength, the angle of incidence, the polarization and the diffraction order. Thus, it is possible to gather a wealth of data thereby allowing the extraction of information about the line profile.
Techniques that utilize the principles of scatterometry and are aimed at the character on of three-dimensional grating structures and the determination of line profiles have been disclosed in numerous publications. Publications, in which diffraction efficiency was measured versus wavelength, include, for example the following:
(1) A. Roger and D. Maystre, J. Opt. Soc. Am, 70 (12), pp. 1483-1495 (1979) and A. Roger and D. Maystre, Optica Acta, 26 (4), pp. 447-460 (1979) describe and systematically analyze the problem of reconstruction of the line profile of a grating from its diffraction properties (the inverse scattering problem). A later article xe2x80x9cGrating Profile Reconstruction by an Inverse Scattering Methodxe2x80x9d, A. Roger and M. Breidne, Optics Comm., 35 (3), pp. 299-302 (1980) discloses how the idea disclosed in the above articles can be experimentally used. The experimental results show that the line profile can be fitted such that the calculated diffraction efficiency will closely match the diffraction efficiency measured as a function of wavelength for xe2x80x9cxe2x88x921xe2x80x9d diffraction order. The comparison of these experimental results with electron microscopy measurement showed a reasonable agreement.
(2) xe2x80x9cReconstruction of the Profile of Gold Wire Gratings: A comparison of Different Methodsxe2x80x9d, H. Lochbihler et. al., Optik, 98 (1), pp. 21-25 (1994) deals with the comparison of the results of several experimental techniques. Both optical transmittance and reflectance efficiencies were measured in the xe2x80x9c0xe2x80x9d order as a function of wavelength. By fitting the measurements to theoretical spectra calculated using diffraction theory, the grating profile was found. Comparison of these results with the results of X-ray diffraction efficiency and electron microscopy showed a good agreement.
(3) Voskovtsova, L. M. et al. , Soviet Journal of Optical Technology 60 (9) pp.617xe2x88x9d19 (1993) studies the properties of gratings fabricated by replica technique. It has been found that the line profile of the hologram diffraction grating differs from the calculated sinusoidal profile. This difference leads to a difference in the spectral diffraction efficiency, an effect that was utilized for process control.
(4) Savitskii, G. M. and Golubenko, I. V., Optics and Spectroscopy 59 (2), pp.2514 (1985) describes a theory for the reflection properties of diffraction gratings with a groove profile which is a trapezoid with rounded comers. Such gratings can be fabricated by a holographic technique with photosensitive materials. It was found that the parameters of the trapezoidal profile, such as the depth of the groove, the width of a flat top and the slope of the side walls, affect the diffraction efficiency of the grating working in the auto collimation regime for the xe2x80x9cxe2x88x921xe2x80x9d order.
(5) Spikhal""skii A. A., Opt Commun 57 (6) pp. 375-379 (1986) presents the analysis of the spectral characteristics of gratings etched into a dielectric material. It has been found that these characteristics can be significantly varied by slightly changing the grating groove profile.
(6) U.S. Pat. No. 5,867,276 discloses a technique for broadband scatterometry, consisting of the illumination of a sample with an incident light beam having a broad spectral composition and detecting a beam of light diffracted from the sample with a spectrometer. The technique is aimed at obtaining the spectrally-resolved diffraction characteristics of the sample for determining the parameters of the sample. The patent suffers from the following drawbacks: the measurements are done in the xe2x80x9c0xe2x80x9d diffraction order which is insensitive to asymmetries in the profile; and the analysis is done using the Neural Network is (N.N.) method, which is sub-optimal by nature for applications requiring a high resolution. Additionally, the method does not take into account the need to focus the light onto a small spot, which is determined by the small area of the test structure allowed in the scribe line.
According to another group of publications, a monochromatic light source (e.g. laser) is utilized, and grating profile parameters are extracted from the measurement of the diffraction efficiency versus incidence angle. Such publications include, for example the following:
(A) S.S.H. Naqvi et al., J. Opt. Soc. Am. A, 11 (9), 2485-2493 (1994) discloses a technique that utilizes measurement of the diffraction efficiency in xe2x80x9c0xe2x80x9d order versus incidence angle to find the height of etched grating. Calculations are based on the Rigorous Coupled Wave Theory (RCWT), initially developed by Moharam and Gaylord and disclosed in M. G. Moharam and T. K. Gaylord, J. Opt. Soc. Am, 71, pp. 811-818 (1981), and several existing statistical techniques for the fitting stage.
(B) Raymond, J. R. et al., SPIE 3050, pp. 476486 (1997) discloses a technique that utilizes a laser beam scanning with a range of angles to measure the diffraction efficiency versus incidence angle and to extract the line profile from the measured data.
(C) U.S. Pat. Nos. 4,710,642 and 5,164,790 disclose optical instruments which require to rotate the sample under test, which is definitely a disadvantage.
(D) U.S. Pat. Nos. 4,999,014; 5,889,593 and 5,703,692 disclose instruments employing angle-dependent intensity measurements without the requirement to rotate the sample. According to these techniques, different optical arrangements are used for providing the changes of the angle of incidence of an illuminating monochromatic beam onto the sample (wafer), without moving the sample. According to U.S. Pat. No. 5,703,692, the measurement is carried out by mechanically scanning the angle of incidence using a rotating block. The main disadvantages of such a technique are as follows: it requires the use of moving parts, the calibration of an angle in a dynamical situation, and has a limited angle range which does not provide enough information allowing accurate extraction of profile. According to U.S. Pat. No. 5,889,593, an optical arrangement includes a first lens that serves for focusing incident light onto a wafer at a range of angles, and a second lens that serves for focusing diffracted light onto a detector array. Although this technique does not need any moving parts, since the measurements are simultaneous, special care has to be taken to destroy coherence and avoid interference between the different light paths. Any suitable component for destroying the coherence always reduces the system resolution, thereby reducing the amount of obtained information.
In a third group of publications, the diffraction efficiency is measured when both wavelength and incidence angle are constant. In this case, information is extracted from the comparison of diffraction efficiency of several orders. This group of publications includes, for example, the following documents:
(I) U.S. Pat. No. 4,330,213 discloses a line-width measurement system using a diffraction grating. In this system, the intensities of first and second order light components are obtained to determine the line-width using empirical formulae.
(II) U.S. Pat. No. 5,361,137 discloses another example of the use of a conventional scatterrometry technique. Here, a set of intensities of the xe2x80x9c1xe2x80x9d or xe2x80x9c2xe2x80x9d diffraction order image of the set of xe2x80x9cfixed-line width and variable-pitch-widthxe2x80x9d test gratings is recorded. From this set of intensities, line-width can be calculated.
Generally speaking, the conventional techniques use the following methodology in order to analyze the measured results:
First, a model is assumed for the grating profile having a number of parameters that uniquely define the profile. The user defines the required model (type of model) and sets the limits and the required resolution for each of the desired parameters.
Second, a spectral library is prepared using an optical model. The spectral library contains the calculated spectra for all possible profiles as defined by the user.
Third, given a measured spectrum, a fitting procedure finds the profile whose calculated spectrum included in the spectral library best matches the measured spectrum.
There is accordingly a need in the aft to facilitate the control of the manufacture of patterned structures by providing a novel method and system for measurements in a patterned structure to determine a line profile of the structure, utilizing the principles of scatterometry.
The term xe2x80x9cpatterned structurexe2x80x9d signifies a structure comprising a plurality of spaced-apart stacks (elements) each including different layers, the pattern being formed by patterned regions and un-patterned regions. The term xe2x80x9cpattern regionxe2x80x9d used herein signifies a region including elements (stacks) having different optical properties, and the term xe2x80x9cun-patterned regionxe2x80x9d signifies a region with substantially uniform optical properties, as compared to the patterned region. Such an un-patterned region is comprised of a single stack including different layers having different optical properties.
The main idea of the present invention is based on obtaining measured data from at least two measurements applied to the same patterned structure (e.g., wafer) in order to achieve both high accuracy and high reliability measurements. The entire measurement procedure is carried out is several steps, taking a different measurement at each step. Analysis, likewise, is performed in several steps, wherein each analysis step utilizes the information obtained in the previous steps. The two measurements could be applied at two different measurement sites located, respectively, in patterned and un-patterned regions. The two measurements may be carried out so as to detect light returned from the structure with different solid angles of propagation, or with different states of polarization.
According to the present invention, at least one parameter of the profile considered in an optical model used for measurements is determined by analyzing at least one preliminary measurement applied to a predetermined site on the structure (wafer). The preliminary measurement is inherently different from further measurements by either the type of site under measurements or the measurement conditions (angle, polarization, wavelength range, diffraction order, etc.). For example, the preliminary measurement utilizes normal incidence of an illuminating beam, while the further measurement utilizes oblique illumination. Data (parameters) obtained through this preliminary measurement is used for optimizing the fitting procedure, thereby improving further measurements applied to other locations on the structure.
Preferably, the parameters obtained through the preliminary measurement include the reflectivity and thickness of at least one layer underneath the uppermost layer. Additionally, the at least one preliminary measurement allows for determining optical constants (i.e., refraction and absorption coefficients n and k) and thickness of the regions of the uppermost layer.
There is thus provided according to one aspect of the present invention, a method of determining a line profile in a patterned structure for controlling a process of manufacture thereof, wherein the patterned structure comprises a plurality of different layers, the pattern in the structure being formed by patterned regions and un-patterned regions, the method comprising the steps of:
carrying out at least first and second measurements, each of the measurements utilizing illumination of the structure with a broad wavelengths band of incident light which is directed on the structure at a certain angle of incidence, detection of spectral characteristics of light returned from the structure, and generation of measured data representative thereof;
analyzing the measured data obtained with the first measurement and determining at least one parameter of the structure; and
analyzing the measured data obtained with the second measurement and utilizing said at least one parameter for determining the profile of the structure.
According to another aspect of the present invention, there is provided a measurement system for determining a line profile in a patterned structure comprising a plurality of different layers, the pattern in the structure being formed by patterned regions and un-pattemed regions, the system comprising a measuring unit including an illumination assembly and a collection-detection assembly, and a control unit coupled to output of the measuring unit, wherein:
the illumination assembly produces incident light of substantially broad wavelengths band directed onto the structure at a certain angle of incidence, and the collection-detection assembly detects spectral characteristics of light returned from the structure and generates measured data representative thereof;
the measuring unit is operable for carrying out at least first and second measurements and generating measured data representative of the detected returned light; and
said control unit is operable to be responsive to the generated measured data for analyzing the measured data obtained with the first measurement to determine at least one parameter of the structure, and utilizing the at least one determined parameter while analyzing the measured data obtained with the second measurement for determining the line profile of the structure.
The scatterometry based measurement technique provides the collection of a large amount of data from each measured profile, e.g., the diffraction efficiency in a large number of different angles or a large number of wavelengths. This richness of data may allow the fitting of the measurements to the results of a multi-parameter model describing the measured profile, thus providing more information than merely stating the CD. This additional information also provides confidence in the results, particularly if the effective number of independent measured values is significantly larger than the number of free parameters in the model. Since exact models describing diffraction from general profiles and in general situations have been developed for years and are known to be of high accuracy, these methods have a good chance of obtaining accurate results.
The system according to the invention can be applied as an integrated metrology tool. In contrast to all conventionally used off-line measurement tools, occupying a large footprint and requiring additional manual operations that slow is down the entire fabrication process and allow only the measurement of samples from each production lot, the system of the present invention may be integrated as part of the production machine, thus allowing full automation of the manufacturing process. For this integration to be possible, the system should be very economical in space.
Additionally, the operation of the system is fast enough, so that every semiconductor wafer in the production line can be measured, allowing closer control over the process. The system of the present invention enables a multi-stage measurement procedure, thereby improving the quality of the entire measurement. The measurement technique according to the invention requires only a small measurement site in accordance with the area constraints, which characterize current lithography.
More specifically, the present invention is Used for process control in the manufacture of semiconductor devices (wafers), e.g., the control of a lithography process, and is therefore described below with respect to this application.